This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 1021015 filed in The Netherlands on Jul. 5, 2002, which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to a method of controlling an inkjet printhead with a substantially closed duct in which ink is present, said duct having an exit opening for the ink. The method includes the steps of actuating an electromechanical transducer so that the pressure in the duct changes in such a manner that an ink drop is ejected from the exit opening, the pressure causing a deformation of the transducer, and, after the end of the actuation, measuring the electrical signal generated by the transducer as a result of the deformation. The present invention also relates to an inkjet printhead suitable for applying the present method and an inkjet printer provided with such a printhead.
2. Background Art
A method of this kind is known from European patent application EP 1 013 453. In this method, the electromechanical transducer, a piezo-electric transducer in this specific case, is energised by applying a voltage or current in pulse form thereto via an actuation circuit. As a result of this energization, the transducer expands in the direction of the duct. As a result, pressure in the duct is suddenly raised. As a result of this pressure increase a drop of ink is ejected from the exit opening. After this drop has left the ink duct, there are, however, still residues of the original pressure wave present because the latter requires some time for a complete damping thereof. This residual pressure wave in turn deforms the piezo-electric transducer so that the latter generates an electric signal which can be measured as a current or voltage. This electric signal is dependent on the state of the duct. For example, if there is an air bubble present in the duct, the damping will be different from what occurs in the case of a completely full duct. Also, a fault in the printhead material around the duct, for example the detachment of a layer of glue between two parts, will also influence this electric signal. In the known process, the piezo-electric transducer is switched into a measuring circuit after the end of the actuation so that the electrical signal can be measured. By comparison with a reference signal, i.e. the signal generated by the transducer of a duct defined as normal, it is then possible to determine whether the duct is in good condition or whether there is a problem which may influence print quality. If a deviation is found, a repair action is carried out, for example flushing the ducts with clean ink. By exact analysis of the signal it is even possible to determine what specific problem has occurred so that a repair action directed towards that problem can be carried out. One example of this is the absence of a wiper for cleaning the front of the printhead, for example because it has broken off. Since the presence of such a wiper, if it cleans the exit side of an ink duct, is visible in the generated electric signal, its absence can also be recorded. In brief, using the known method, an ink duct, and anything in the printer which also determines the state in the ink duct, it is possible to continuously check for proper operation and make repairs if a problem occurs. In this way a permanently good print quality can be achieved.
The known method, however, has a number of significant disadvantages. Firstly, if a deviation of the electric signal is found, it will in most cases result in a repair action even if this is unnecessary or illogical. It is often expensive, because such an action relates, for example, to rinsing the printhead with clean ink or even replacing the entire printhead. This also affects productivity, because no receiving materials can be printed during the repair action. In addition, the known method has problems with compensating for changes which are small or occur gradually but which do influence the print quality. For example, as a result of ageing, the coefficient of expansion of the piezo-electric transducer can change slowly. Up to a specific threshold value, no repairs will be carried out in the known method while there may nevertheless be an appreciable influence on print quality. This disadvantage occurs, for example, even if the printhead is provided with new ink, i.e. ink from a different batch. If this ink were to give rise to another electric signal, something which is quite possible because the viscosity of the ink has a significant influence on the pressure curve in the duct, and said change is lower than the threshold value, then no action is taken while the print quality may well be influenced. If, however, the measured signal does differ from the reference signal sufficiently, then a repair action in principle is illogical because flushing with ink will not result in a different ink in the duct. In the known method, this problem can be compensated for by providing the printer, for example in the printhead, with sensors to measure all types of variables affecting the pressure build up in the ink duct, such as the above-mentioned ink viscosity. Depending on the measured value for one or more of these variables, then a different reference signal can be selected. The disadvantage of this, however, is that sensors must be incorporated in the inkjet printer. Sensors, however, are expensive and not always easy to implement. In addition, the number of sensors that can be used is limited, simply because there is frequently no room for a large number of sensors. Thus usually sensors are provided solely to measure the temperature of the printhead, the pressure in the ink duct and the level of the ink in an ink reservoir connected to the ink duct. Since there are many more variables which influence the pressure build-up in a duct as a result of actuation of the transducer, this known method provides only a limited solution of the above problems.